Conventionally, it has been known that examples of a vehicle (automobile) brake system include those having a servo unit such as a negative pressure booster and an oil pressure booster. In addition, an electric servo unit using an electric motor as a power source has recently been known (see, for example, Patent Literature 1).
Patent Literature 1 discloses an electric servo unit including: a main piston which moves back and forth by brake pedal operation; a tubular booster piston which is fitted to the main piston in such a manner as to enable its position to be relatively changed; and an electric motor which moves the booster piston back and forth.
This electric servo unit uses the main piston and the booster piston as pistons of a master cylinder. The front end of each piston faces a pressure chamber of the master cylinder. An operator inputs a thrust to the main piston by using a brake pedal. Also, the electric motor inputs a booster thrust to the booster piston. Accordingly, these thrusts can generate a brake hydraulic pressure in the master cylinder.